GREENSBORO, NC -- Kids ask the best questions. In the last year of 2WTK, we've learned about how bricks are made, what it's like on the International Space Station, and where marshmallows came from, all because kids asked questions. Today's question is no different.

Jaden asked, "How you construct a bridge without it falling in?"

North Carolina maintains 13,638 bridges. And the keys to constructing them or any bridge is "bats". No, not the animals. B-A-T-S.Beams, Arches, Trusses and Suspensions.Those four things determine the bridge design.

Whether it's a cable bridge, suspension bridge or a beam bridge like the ones we drive on every day, BATS plays a part. . But as for how they stay standing? Greensboro DOT engineer Arnold Baldwin says it all starts with the right foundation.

"We analyze the dirt, the material, at the lab that we have in Raleigh, they'll determine the length, type of material we have, that'll determine the rock that'll determine the length of the pile that we drive, to know what the burden is to start out with a good foundation for the bridge."

Next the crews place the rebar and pour the concrete. "And we use wooden forms which is usually made out of 2 by 4s and plywood, so the form those together and tie them together with snap ties so then it just holds the forms together so we can pour the concrete in and it won't push them back out."

The colder the temperature, you need it in the forms longer, that could take up to 36 hours or 3 days. On a hot day like today it could be the next day, 24 hours, to pull the forms, it would be set.

If you think it's cool to be able to build a bridge, study! "There's a lot of math as far as figuring the calculations. You've got a lot of algebra, geometry and stuff, lot of math involved."

You want to make sure those calculations are right, because it's everyone's safety!

What is the longest US bridge? Lake Ponchartrain Causeway in Louisiana is 23.85 miles long!